Wireless local area networks (WLANs) have become increasingly popular in various scenarios, such as, in homes, offices, and public places. The first WLAN IEEE standard to become accepted in the commercial market was 802.11b, which specifies encoding techniques that provide for raw data rates up to 11 Mbps using a modulation technique called “Complementary Code Keying” (CCK). The 802.11b standard also supports a modulation technique called “Direct-Sequence Spread Spectrum” (DSSS) from the original 802.11 standard. Later another WLAN IEEE 802.11a standard appears with using a more efficient transmission method called Orthogonal Frequency Division Multiplexing (OFDM). OFDM, as implemented in 802.11a, enabled raw data rates up to 54 Mbps. Despite its higher data rates, the 802.11a standard was not as popular as the 802.11b standard because the 802.11a standard resides on an incompatible radio frequency band: 5 GHz for 802.11a versus 2.4 GHz for 802.11b. In June 2003, the IEEE ratified the 802.11g standard, which applied OFDM modulation to the 2.4-GHz band. The 802.11g standard combined the raw data rates up to 54 Mbps on the same radio frequency as the already popular 802.11b standard. A similar scenario to the draft 802.11g phenomenon is now unfolding with 802.11n. The industry came to a substantive agreement with regard to the features to be included in the high throughput 802.11n standard in early 2006.
In 802.11 WLANs, a carrier sense multiple access with collision avoidance (CSMA/CA) is used where a carrier sense multiple access with collision detection (CSMA/CD) cannot be implemented due to the nature of the channel. One reason, among others, is that wireless devices have difficulties in listening while sending data for medium access; therefore, collision detection is less likely to be achieved. In CSMA/CA, however, if the channel is determined to be busy before transmission, the transmission is deferred for a random interval. This reduces the probability of collisions on the channel.
Accordingly, the use of simple and robust CSMA/CA for medium access has contributed to the success of the 802.11 standards. However, the original CSMA/CA in 802.11 standards has some drawbacks such as high overhead, low throughput, without quality of service (QoS), etc. Although the 802.11e standard could support QoS and its media access control (MAC) also improves channel efficiency using the Block Ack technique, this mechanism is quite complex in the 802.11e standard because the 802.11e standard uses an explicit setup and tear-down procedure. Similarly, the IEEE proposed 802.11n standard draft now also can not opportunistically use high-rate hosts to improve overall channel throughput.